The United States is experiencing a demographic sea change owing to the rapidly growing elderly and 'Baby Boomer'populations. Our astonishing biomedical advances in the last half-century have greatly increased our life expectancy. But as a consequence of living longer, our population now faces an uptick in the incidence of neurodegenerative diseases. These truly disastrous disorders include Alzheimer's, Huntington's, Parkinson's, amyotrophic lateral sclerosis (ALS) and the frontal temporal dementias. Interestingly, though disparate in their pathophysiology, many of these diseases share a common theme manifest in the accumulation of insoluble protein aggregates in the brain and nervous system. Deciphering the mechanisms causing these proteins to misfold and aggregate and identifying the genes and cellular pathways affected by misfolded human disease proteins will aid the development of new therapeutic approaches, which we are in dire need of. The protein TDP-43 was recently identified as the major disease protein in pathological inclusions in both ALS and frontal temporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U). Moreover, mutations in the TDP-43 gene have now been identified in sporadic and familial ALS patients. Pathology and genetics both converge on TDP-43 being central to the pathogenesis of these diseases. The study of tau, alpha-synuclein, and amyloid beta in neurodegenerative disorders has revolutionized our understanding of their respective disease mechanisms. A similar intense effort to understand TDP-43 biology and its role in pathology will be beneficial to the development of therapies to treat ALS and FTLD-U. Nothing is yet known about how TDP-43 causes disease (loss- or gain-of-function mechanism). We have generated in vitro and in vivo TDP-43 proteinopathy models to explore TDP-43 and our preliminary data demonstrate: 1) a critical role for the RNA recognition motif and carboxy-terminal region of TDP-43 in mediating aggregation and cellular toxicity, 2) increased aggregation and toxicity caused by a disease-linked TDP-43 mutation, and 3) a pilot genetic screen identified multiple RNA binding proteins as potent toxicity modifiers. We hypothesize that TDP-43 causes disease by a toxic gain-of-function mechanism involving RNA binding. Thus, with the goal to define TDP-43 disease mechanisms from multiple angles we propose three Specific Aims: 1) Directly testing the requirements for RNA binding by TDP-43 on aggregation and toxicity and testing the effect of ALS-linked TDP-43 mutations in vitro and in vivo;2) Performing two genome wide screens for genetic modifiers of TDP-43 aggregation and toxicity, and;3) Collaborating with a team of experts to develop animal models (zebrafish, Drosophila, C. elegans) to explore TDP-43 disease mechanisms and validating modifier genes from our genetic screens.